It is a joint portion of rails where it is most likely to be damaged and cost the most for the maintenance. Further, the joint portion is a main source of noise and vibration generated during train passage. Since the speeding up of passenger railway operations and the increase in loads of freight railways are promoted in many countries, the following technology is generalized: the rail joints that cause the above problems are welded such that the rails are formed into a continuous long rail.
With reference to FIGS. 1A to 1D, there are described names for a weld zone of the long rail and a cross section of the rail. FIG. 1A is a side view of a weld zone in a longitudinal direction. The long rail is produced by welding at least two rails. Accordingly, the long rail includes a weld zone 7. A bead 8 is present on the weld zone 7.
FIG. 1B is a cross-sectional view along the line A-A perpendicular to a longitudinal direction of a rail at a welding center Q. The rail has a head portion 1 that is an upper part of the rail which comes in contact with wheels, a foot portion 3 that is a lower part of the rail which is in contact with ties, and a rail web portion 2 that is a perpendicular part between the head portion 1 and the foot portion 3. Further, a point 4, which is the highest point of the head portion may be referred to as head-top portion, a top surface 5 of the foot portion may be referred to as foot-top, and a back surface 6 of the foot portion may be referred to as sole or base.
FIG. 1C is a cross-sectional view along the line B-B parallel and vertical to the longitudinal direction of the rail, which includes the welding center Q. There is a region which is heated to more than or equal to an A1 transformation point by welding, that is, there are boundary lines X of a heat-affected zone (hereinafter, referred to as HAZ) at both sides of the welding center Q.
FIG. 1D is a cross-sectional view along the line B-B in the case of melt welding such as thermite welding and enclosed arc welding. There are melting boundaries Z at both sides of the welding center Q, and the inside thereof is a weld metal.
Next, a method of welding rails is described. There are four examples of main methods of welding rails: flash butt welding (for example, Patent Document 1), gas pressure welding (for example, Patent Document 2), enclosed arc welding (for example, Patent Document 3), and thermite welding (for example, Patent Document 4).
As shown in FIGS. 2A to 2C, the flash butt welding is a welding method involving applying voltage through electrodes 9 to welding materials 10 placed face to face, causing an arc to be formed between end faces of the welding materials to melt the end faces, and, at the time point at which the welding materials are sufficiently heated, bonding the welding materials by pressurizing in an axial direction the materials.
The thermite welding is a method involving causing the welding materials 10 to be placed face to face with a gap of 20 to 30 mm therebetween, surrounding the gap part with a mold, producing molten steel resulting from a reaction of aluminum with iron oxide, the reaction taking place inside a crucible set at an upper part of the mold, and injecting the molten steel into the mold to melt end faces of rails and weld the end faces to each other.
The gas pressure welding is a method involving heating, in the state of bonding faces being pressurized, the welding materials in the vicinity of the bonding faces from the side surfaces using a burner, and pressure welding the bonding faces at a high temperature. The vicinity of the weld zone is expanded and deformed by the pressurization. The expanded portion is removed by a trimmer.
The enclosed arc welding is a manual arc welding method involving causing the welding materials to be placed face to face with a gap of 10 to 20 mm therebetween, surrounding the gap with a backing strip and a side strip, and heaping up the weld metal at the gap with a welding rod.
In a rail weld zone, there are cases where a fatigue crack is generated from a neutral axis of the rail web portion of the rail weld zone, particularly in heavy load freight railways and in a cold district, the fatigue crack causes a brittle fracture, and thus increases the frequency of replacing the rails. FIG. 3A and FIG. 3B show a form of the damage.
That is, FIG. 3A is a view of a horizontal crack generation state of the rail web portion viewed from the side face of the rail. A fatigue crack 22 is generated in the horizontal direction from a point of a weld defect near a reinforcement of weld in the vicinity of the neutral axis of the rail web portion, then a brittle crack 23 penetrates the thickness of the rail web portion, and after that, one end of the crack propagates towards the head-top side and the other end of the crack propagates towards the base side. Although there is the case where an origination of the fracture is the weld defect, the fracture may also be generated from the surface of the weld zone when there is no defect.
FIG. 3B shows a state where the site at which the horizontal crack is generated in the rail web portion is cut, and a crack surface is opened and viewed from the rail head-top side. The following state can be seen: the fatigue crack 22 is generated from near the center of the rail web portion of the rail weld zone; and then the brittle crack 23 penetrates the thickness of the rail web portion.
As described in Non-Patent Document 2, it is considered that the generation of the fatigue crack is affected by not only an external load condition but also residual stress in the materials. FIG. 4 shows results measured by the inventors of the present application of the residual stress distribution within a cross section in a circumferential direction, the cross section forming a right angle with the longitudinal direction of the rail at the welding center Q of the rail weld zone. The fatigue crack 22 is generated and propagated, because large tensile residual stress is generated by the welding in the vicinity of the rail web portion 2 of the weld zone in the circumferential direction of the rail, that is, in the vertical direction, and a load is repeatedly applied each time trains pass. In order to prevent such damage, it is desirable to prevent the weld defect which is a damage origination, and also to render the defect harmless even if there is the defect. From this viewpoint, it is desirable that the residual stress in the vertical direction applied to the rail web portion be smaller. According to a fatigue test performed by the present inventors, it is desirable that, for reducing the frequency of the generation of the fatigue crack, the residual stress in the vertical direction be 350 MPa or less.
A railway track is formed of broken stone ballasts, ties, devices each fastening a rail and a tie, and rails. During the passage of trains on the rail, a load distributed from many wheels of the trains are applied to the railway track.
In considering a cause for generating the damage, it is necessary to consider a load condition from the wheels with respect to the rail weld zone. The most typical states of the relationship between a rail and ties supporting the rail are: a state in which a vertical load is directly applied to the rail a wheel passes immediately above the tie; and a state in which the wheel passes an interval between ties. When a long rail produced by the welding performed in a manufacturing facility is placed at an actual location, it is only by chance that a position of a weld zone and a position of a tie meet with each other. It is considered that, in one long rail having a length of several hundred meters, there are several parts at which a position of a tie and a position of a weld zone meet with each other.
FIG. 5 shows a case where a wheel 25 passes immediately above a tie 24. In this case, the largest stress is generated at the rail web portion 2 which has a small cross sectional area. Although the stress in this case is compressive stress, the stress at the rail web portion 2 of the weld zone 7 has the excessively large tensile residual stress, and hence, the stress is substantially in a repetitive stress state in a tensile region.
Further, it is considered that the state in which the wheel passes an interval between the ties is the other typical loaded state, and as shown in FIG. 6, a load that bends the rail by pushing from an upper part is added. In this loaded state, a rail head portion is compressed in a longitudinal direction, a rail foot portion is pulled in the longitudinal direction, and the rail web portion is neutral. However, in winter, shrinkage stress is generated in the longitudinal direction of the rail in many cases, and tensile stress may be applied repeatedly at a position where a height of the rail web portion is low. When the residual stress in the longitudinal direction is applied in addition to the shrinkage stress and the tensile stress, there is a risk in winter that a fatigue crack in a direction that forms a right angle with the longitudinal direction of the rail, that is, in the vertical direction, may be generated at the rail web portion, the fatigue crack being attributed to the stress in the longitudinal direction of the rail.
Note that the tensile stress is applied to the rail foot portion each time the wheel passes. However, as shown in Non-Patent Document 2, the residual stress of a flash butt weld zone in the longitudinal direction is in a large compressive stress state in the rail sole portion. FIG. 7 shows results measured by the inventors of the present application. Accordingly, the tensile stress applied to the rail foot portion through the passage of trains and the residual stress offset each other, and thereby forming a compressed region, which gives an advantage that fatigue cracks are not easily generated.
In order to improve durability of the long rail, it is necessary to suppress the generation of the fatigue crack from the rail web portion of the weld zone and to achieve the fatigue-resistant characteristics of those portions.
In order to prevent the damage to the rail web portion, the inventions of Patent Documents 5 and 6 each disclose a method of improving the fatigue resistance of the rail weld zone by controlling the residual stress using rapid cooling of the head portion and the rail web portion of the rail weld zone or the entire rail weld zone, which is in a high-temperature state attributed to welding heat or heat transferred from the outside, and reducing tensile residual stress that is generated at the rail web portion of the rail weld zone in the vertical direction or converting the tensile residual stress into compressive stress. According to those inventions, it has become possible to largely reduce the generation of the fatigue crack from the rail web portion.
When the method of rapidly cooling the head portion and the rail web portion after the welding as described in Patent Documents 5 and 6 is performed, Non-Patent Document 1 shows that the residual stress that is generated at the rail web portion in the vertical direction [o1] is reduced, and thus, the generation of the fatigue crack in the rail web portion can be suppressed. However, according to this method, it is illustrated that the residual stress of the sole portion in the longitudinal direction is converted into tensile stress. In recent years, there has been a tendency that weight of freight cars in heavy freight railways is increasing, and as a result thereof, a bending load applied to the sole portion is increased. The sole portion is pulled in the longitudinal direction of the rail due to the bending load, and a bending fatigue strength of this part is strongly influenced by the residual stress in the longitudinal direction. If the residual stress of the sole portion in the longitudinal direction is converted into tensile stress due to the cooling method of Patent Documents 5 and 6, there is a risk that bending fatigue resistance may be lowered.
As other techniques that improve the fatigue strength of the rail weld zone, there are given a method using shot peening as described in, for example, Patent Document 7, and methods using hammer peening, grinder treatment, and TIG dressing.
Further, Patent Document 8 shows a method of reducing the residual stress by reheating the weld zone with a gas burner. There is shown a possibility that this method may reduce the residual stress, but Patent Document 8 does not show an appropriate reheating region that is assumed to be different for each welding method, and it is not necessarily sufficient for preventing fatigue damage.